Meta

Tag: Inner Mongolia

Like most children who have spent holidays on a beach somewhere, both my wife and I have memories of playing in sand dunes, I in Norfolk
she along Italy’s Adriatic coast.
Then, after we met and started traveling the world we visited a number of large-scale sand dune systems, far away from any sea. The first we saw, at the start of our lives together, were the sand dunes of Death Valley.
On a business trip a few years ago, I visited the sand dunes of Inner Mongolia which are remorselessly engulfing farmland; I commented on these in an earlier post.
And then there were the sand dunes of Namibia, which we visited one Christmas some ten years ago with our children.

The most awesome of these dunes were a dull red, with the biggest towering over us.

But the blindingly white dunes of the White Sands National Monument in New Mexico, which we visited last week with our daughter and her boyfriend, are in a class of their own. The eerie beauty of these dunes has led better photographers than I to surpass themselves, and I have shamelessly pinched some of the best of their photos to insert here.
We walked in a big circle, from dune crest to dune crest, marveling at the vistas before us of undulating whiteness, all the way, so it seemed, to the surrounding mountain ranges.
I’ve been careful not to refer to these dunes as sand dunes, because they are not sand as we normally understand that term, that is to say silicate. These are dunes of gypsum (calcium sulphate to the chemically inclined of my readers).

In the morning, we had visited Lake Lucero, which lies to the southwest of the dunes. It is an evanescent lake; it appears during the rainy season in late summer and is gone by the time the windy season in the spring rolls around. When we visited it, there was no sign of any water.

Here we could see how the gypsum “sand” had been created. It all started 24,000 years ago, when a new ice age started and the climate in this corner of New Mexico began to be much wetter than it is today. For nigh on 14,000 years frequent rains lashed the nearby San Andres and Sacramento mountain ranges. The rainwater nibbled away at strata of gypsum which had been exposed by the mountains’ uplifting, and streams carried the dissolved gypsum into a lake at the foot of the mountains. This lake has been named Lake Otero. The lake had no exit, so it grew in size until the water evaporating balanced the stream water entering. But the gypsum (and other salts) carried into the lake remained and slowly concentrated. Then, some 10,000 years ago, the ice age came to an end, and the climate here became dryer. Less rainwater fell on the mountains, streams got smaller or disappeared, and Lake Otero began to shrink. As it shrunk, the dissolved gypsum became ever more concentrated until finally the lake water was saturated. The gypsum started precipitating out of the lake water, forming huge crystals of selenite in the process, which then settled onto the lake’s bed. This picture shows a very pure crystal of selenite, which is colourless.

But during our ramble along the shores of Lake Lucero we saw selenite crystals in their more natural state, jutting out of the ground. They were various shades of brown; other substances that were present in the lake water have colored the crystals.

And still Lake Otero kept shrinking, until nothing but Lake Lucero – sometimes there, often not – was left.

With the climate change of 10,000 years ago came strong winds. For the last 10,000 years they have been scouring the alkaline flats left bare when Lake Otero disappeared. They first carried away the thin crust of clay and fine particles, which exposed the selenite. Freezing and thawing cycles went to work on the crystals, breaking them along their weakest plane. The process continues to this day. We saw these crystals of selenite down at Lake Lucero being slowly split open.

The shores of Lake Lucero are littered with fragments of selenite crystals, broken up by wind, frost and heat.

On and on went the work of breaking down the crystals until they had become flakes light enough to be carried along by the prevailing southwesterly winds. As the flakes bowled along over the alkaline flats, tumbling over and over, they cracked and crumbled further until only small sand-like grains were left.

The winds have pushed the grains up into the dunes that we see today. And all that tumbling has so scratched and scarred and pitted the surface of the grains that they reflect back sunlight in all its wavelengths so that we see them as intensely white.
Unbeknownst to us, my wife and I have been living with selenite around us in the apartment in Milan, although in this case in the form of desert roses.
My mother-in-law picked them up in Algeria, where she visited some of the oases south of the Atlas Mountains and walked the dunes of the Saharan desert, like these at the Biskra oasis.
Another dune system for my wife and I to visit – once Isis no longer roams the Sahara desert, kidnapping and beheading hapless tourists.